What does Mercie mean in English?

Excuse me, could you possibly elaborate on the meaning of the word "Mercie" in the English language? I've come across it in a few contexts, but I'm not entirely sure of its precise significance. Could you provide a clear and concise definition that encapsulates its CORE meaning? It would be greatly appreciated if you could offer some insight into its etymology or usage as well. Thank you for your time and assistance.